Coinage-Metal-Stuffed Eu₉Cd₄Sb₉: Metallic Compounds with Anomalous Low Thermal Conductivities

Abstract

The synthesis and transport properties of the family of coinage metal-stuffed Zintl compounds, Eu9Cd4–x CM2+x–y □ y Sb9 (CM = coinage metal, □ = vacancies), is presented as a function of coinage metal substitution. Eu9Cd4–x CM2+x–y □ y Sb9 compounds are shown to be rare examples of metallic Zintl phases with low thermal conductivities. While the lattice thermal conductivity is low, which is attributed to the complex structure and presence of interstitials, the electronic contribution to thermal conductivity is also low. In these p-type compounds, the carriers transmit less heat than expected, based on the Wiedemann–Franz law and metallic conduction, κ e = L 0 T/ρ. Density functional theory (DFT) calculations indicate that the Fermi level resides in a pseudo-gap, which is consistent with the metallic description of the properties. While the contribution from the interstitial CM states to the Fermi level is small, the interstitial CMs are required to tune the position of the Fermi level. Analysis of the topology of electron localization function (ELF) basins reveals the multicenter Eu−Cd­(CM)−Sb interactions, as the Eu and Sb states have the largest contribution at the top of the valence band. Regardless of the success of the Zintl concept in the rationalization of the properties, the representation of the CM-stuffed Eu9Cd4Sb9 structure as Eu cations encapsulated into a polyanionic (Cd/Cu)Sb network is oversimplified and underestimates the importance of the Eu–Sb bonding interactions. These results provide motivation to search for more efficient thermoelectric materials among complex metallic structures that can offer less electronic thermal conductivity without deteriorating the electrical conductivity.

Achieving zT > 1 in Inexpensive Zintl Phase Ca9Zn4+xSb9 by Phase Boundary Mapping

“…However, the κ L of the Ca 9 Zn 4+ x Sb 9 sample and also the κ of the samples with x ≥ 0.6 are found to be even below the estimated κ glass . In fact, almost all phases isotypic to the Ca 9 Zn 4+ x Sb 9 structure have extremely low κ L . To rule out the possibility that the calculated Dulong–Petit heat capacity (≈0.32 J g −1 K −1 ) underestimates the real value of C p , we measured C p up to 623 K (as shown in Figure S5 in the Supporting Information).…”

“…Possible temperature dependency of the speed of sound could also lead to an overestimation of κ glass at high temperature. Although further investigation is required to determine the origin of such an ultralow thermal conductivity, most notably in other A 9 M 4+ x Pn 9 phases, extremely low κ L could be due to anomalously low Lorenz factors or electron–phonon interactions as demonstrated in Eu 9 Cd 4− x CM 2+ x − y □ y Sb 9 compounds (where CM is a coinage metal, e.g., Au, Ag, and Cu) …”

“…A 9 M 4+ x Pn 9 (A = Yb, Eu, Ca, Sr; M = Mn, Zn, Cd; Pn = Sb, Bi), often referred to as the 9‐4‐9 system, are Zintl phases recently identified with promising thermoelectric performance . The crystal structure was originally reported in 1970s, but the existence of partially occupied interstitial sites, whose occupancy is indicated by x in the formula, was not recognized until the work of Bobev et al in 2004 ( Figure a) .…”

Enclathration of X@La4 Tetrahedra in Channels of Zn–P Frameworks in La3Zn4P6X (X = Cl, Br)

“…In compounds with high rare-earth content, ~50%, non-isotropic bonding interactions involving rare-earth elements were detected by ELF and electron density distribution analyses. [52,53] However, it is uncommon for compounds with low rareearth content, 21% for La 3 Zn 4 P 6 Cl, to exhibit such type of chemical bonding. For analogues of the title compounds containing magnetic rare-earth metals, this bonding scheme may lead to long-range magnetic interactions.…”

Sr/Eu cation doping and thermoelectric properties of the compound Ca<sub>9</sub>Zn<sub>4.5</sub><sub>−</sub><sub><italic>δ</italic></sub>Sb<sub>9</sub> (0<<italic>δ</italic><0.5)

“…Zintl相有着复杂的晶体结构和较低热导率的特点, 这使其在高温热电能源发电领域表现出了较高的潜 能 [19,20] . 在过去的数十年中, 大量Zintl相被研究和扩展 [34,35] , Eu 9 Cd 4 Sb 9 [36,37] 和Ca 9 Zn 4.5−x Cu x Sb 9 [33] 化合物材料中取得的最大ZT值可以达到0.7. 最近报道 的一篇Ca 9 Zn 4.5−δ Sb 9 研究中, 通过第二相固溶的方法改 变了化合物中化学势能进而改变了化合物成分, 增大 了过渡金属Zn的含量, 最终在875 K得到了1.1的ZT 值 [38] .…”